High-Performance Alkaline Organic Redox Flow Batteries Based on
2-Hydroxy-3-carboxy-1,4-naphthoquinoneHigh-Performance Alkaline
Organic Redox Flow Batteries Based on 2Hydroxy-3-carboxy-
1,4-naphthoquinone Caixing Wang,†,⊥ Zhen Yang,‡,⊥ Yanrong Wang,†
Peiyang Zhao,† Wen Yan,† Guoyin Zhu,† Lianbo Ma,†
Bo Yu,† Lei Wang,† Guigen Li,*,‡,§ Jie Liu,†, and Zhong
Jin*,†
†Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry
and Chemical Engineering, Nanjing University, Nanjing, Jiangsu
210093, China ‡Institute of Chemistry & BioMedical Sciences,
Nanjing University, Nanjing, Jiangsu 210023, China §Department of
Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas
79409-1061, United States Department of Chemistry, Duke University,
Durham, North Carolina 27708, United States
*S Supporting Information
ABSTRACT: Aqueous redox flow batteries (ARFBs) based on the
electrolyte solutions of redox-active organic molecules are very
attractive for the application of large-scale electrochemical
energy storage. We propose a high-performance ARFB system utilizing
2- hydroxy-3-carboxy-1,4-naphthoquinone (2,3-HCNQ) and K4Fe- (CN)6
as the anolyte and catholyte active species, respectively. The
2,3-HCNQ molecule exhibits high solubility and can carry out a
reversible two-electron redox process with rapid redox kinetics.
The assembled 2,3-HCNQ/K4Fe(CN)6 ARFB delivered a cell voltage of
1.02 V and realized a peak power density of 0.255 W cm−2. The 2,3-
HCNQ/K4Fe(CN)6 ARFB can be stably operated at a current density of
100 mA cm−2 for long-term cycling (with a capacity retention of
∼94.7% after 100 cycles).
Redox flow batteries (RFBs) present a promising prospect in
grid-scale energy storage, especially for mitigating the
intermittent fluctuation of renewable
energy sources, such as solar and wind power plants.1−4 The
operation of RFBs depends on the reversible electrochemical
reactions of redox-active species dispersed in liquid-phase
anolytes and/or catholytes.5−8 The modular design of redox pairs
should take into consideration several crucial properties, such as
solubility, voltage window, reaction kinetics, cycling stability,
etc. Over the past decade, great efforts have been made to develop
aqueous redox flow batteries (ARFBs) that adopt water-soluble
organic molecules in electrolytes.9−14
Several classes of organic redox-active species, including quinone,
viologen, alloxazine, 2,2,6,6-tetramethylpiperidinooxy (TEMPO), and
their derivatives, have attracted considerable attention for the
application in ARFBs, owing to the rapid redox kinetics, low
corrosivity, high solubility, and low cost.11−18 It is also known
that organic molecules have excellent structural diversity;
therefore, the electrochemical properties can be easily tailored by
the modification of different functional groups.19−29 Previous
works suggested that the electron-donating groups can lower the
redox potential, while the electron-withdrawing groups can elevate
the redox
potential, endowing the organic ARFBs with more flexible voltage
regulation than traditional ARFBs.12,14,19 Generally, the organic
molecules often existed in negatively charged forms either in
acidic or alkaline solutions during the whole charge− discharge
process, so the crossover issue of active species through the
separator can be overcome by employing a cation separator. It
should be noticed that the redox species tending to release protons
have higher solubility either in acid or alkaline solution, thus
contributing to higher energy density and power density. For
example, anthraquinone-sulfonic acid derived from anthraquinone can
be dissolved with 1.0 M concentration in acid conditions.13 In
addition, the β-OH groups on the anthraquinone can significantly
increase the solubility in the alkaline solution for improved
proton dissociation in comparison with the α-OH
groups.14,27,29
Alloxazine-derived redox molecules also show improved solubility in
the alkaline solution after the introduction of a carboxyl group or
hydrotropic agent.17,19 Given the fact that a
Received: July 22, 2018 Accepted: September 13, 2018 Published:
September 13, 2018
Letter http://pubs.acs.org/journal/aelccpCite This: ACS Energy
Lett. 2018, 3, 2404−2409
© 2018 American Chemical Society 2404 DOI:
10.1021/acsenergylett.8b01296 ACS Energy Lett. 2018, 3,
2404−2409
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large number of redox-active organic species have a relatively
narrow voltage window and low solubility in water that may result
in low volumetric energy density, it is vital to design novel
organic molecular structures to fully satisfy the demands of ARFBs.
The derivatives of naphthoquinone often serve as electron
acceptors in biochemical electron transport processes in organisms,
such as photosynthesis30−32 and aerobic respira- tion.33,34 Ding et
al. reported the use of naphthquinone derivates in organic RFBs.35
Recently, naphthquinone functionalized with diethylene glycol
monomethyl ether groups was reported to exhibit a high energy
density (264 W h L−1) in solvent-free organic redox flow
batteries.36 However, most quinone derivatives exhibit low
solubility in water, which would limit the application in ARFBs.
2-Hydroxy-1,4- naphthoquinone (2-HNQ), also well-known as
“lawsone”, is a nature-abundant and low-cost red-orange dye that
can be extracted from the leaves of henna trees and the flowers of
water hyacinths. Each 2-HNQ molecule can undergo a reversible
two-electron redox conversion between the quinone form and the
phenol form, offering a potential of −0.70 V vs Ag/AgCl; thus, it
can be used as a potential anolyte in ARFBs.29 However, owing to
the low solubility of 2-HNQ (0.48 M in alkaline solution) and the
possible side reactions with O2, the previous ARFB based on 2-HNQ
anolyte (0.2 M) showed a low volumetric capacity and suffered from
large capacity loss and poor cycling stability (<50% capacity
retention per cycle at the low current density of 8 mA cm−2).
Therefore, 2-HNQ is inadequate to entirely fulfill the requirements
for the application in ARFBs. Here we report a mild and low-cost
synthesis route to obtain
a new negative redox-active naphthoquinone derivative organic
material: 2-hydroxy-3-carboxy-1,4-naphthoquinone (2,3- HCNQ). The
introduction of the hydroxyl and carboxyl groups decreases the
redox potential of 2,3-HCNQ to −0.73 V vs Ag/AgCl and also greatly
increases its solubility to 1.2 M in alkaline solution. The ARFB
model based on alkaline 2,3- HCNQ anolyte matched with K4Fe(CN)6
catholyte in 1.0 M KOH (Figure 1) exhibits a cell voltage of 1.02
V, which is higher than those of acidic all-quinone or
quinone-bromine ARFBs11−13 and comparable to that of the alkaline
ARFBs based on flavin mononucleotide (FMN) and 1,8-dihydroxyan-
thraquinone (1,8-DHAQ).17,27 The 2,3-HCNQ molecule also exhibits
other superior properties, including rapid redox kinetics, high
ionization constants, and good electrochemical and thermal
stability. Promoted by the rapid redox kinetics of 2,3-HCNQ, the
2,3-HCNQ/K4Fe(CN)6 ARFB can be stably operated at a current density
of 100 mA cm−2 for long-term cycling (with a capacity retention of
∼94.7% after 100 cycles), which is comparable with that of existing
alkaline and neutral ARFBs (Table S1).14−27
Through a facile and low-cost total synthesis route including a
two-step cycloaddition step and an ester hydrolysis step, 2,3- HCNQ
was synthesized with an overall yield of 70% (Figure 2a). Moreover,
the synthesis cost of 2,3-HCNQ is very low, as estimated in Figure
S1. Nuclear magnetic resonance (NMR) and liquid
chromatography-tandem mass spectrometry (LC- MS) spectra confirm
the formation of 2,3-HCNQ (Figures S2−S4). The cyclic voltammetry
(CV) curves show that 2,3- HCNQ undergoes a reversible redox
process in 1.0 M KOH on the glassy carbon electrode (GCE) (Figure
2b), corresponding to a formal potential (the average value of the
anodic peak and cathodic peak potentials) of −0.73 V vs Ag/
AgCl electrode (−0.52 V vs standard hydrogen electrode, SHE). As
the sweep rates increases from 25 to 100 mV s−1, the
anodic−cathodic peak differences are still kept at ∼48 mV. Figure
2c shows the peak currents versus square roots of scan rates
derivated from Figure 2b. According to the Randles− Sevcik equation
(eq 1)
Figure 1. Cell configuration of ARFB based on 2,3-HCNQ anolyte and
K4Fe(CN)6 catholyte. Both the anions are shown in ball-and- stick
models (OH− is omitted for clarity). During the charge process
(blue arrows), 2,3-HCNQ is reduced to its phenol form, while
ferrocyanide (pink balls for Fe(II)) is oxidized to ferricyanide
(yellow balls for Fe(III)), and K+ ions (blue balls) shift from
catholyte to anolyte to balance the anion concentration. The
discharge process is just the opposite (red arrows).
Figure 2. (a) Synthesis route of 2,3-HCNQ. (b) CV curves of 2 mM
2,3-HCNQ aqueous solution in 1.0 M KOH at different scan rates. (c)
Peak currents vs square roots of scan rates. (d) Titration curve of
2,3-HCNQ (2.0 mL saturated 2,3-HCNQ solution (at pH 3.98) diluted
to 40 mL and then titrated by 1.026 mM KOH standard solution). (e)
UV−vis absorption spectra of 0.1 mM 2,3- HCNQ solution at pH 7 and
pH 14.
ACS Energy Letters Letter
i An D Cv2.69 10p 5 3/2 1/2 1/2= × × (1)
the slope ratio of cathodic and anodic scans is calculated to be
−1.04, which implies the 2,3-HCNQ and reduced 2,3-HCNQ molecules
have similar diffusion coefficients. To understand the influence of
the additional carboxyl
group, the ionization constants of 2,3-HCNQ were measured by the
titration of standard KOH solution, and pKa1 and pKa2 were
calculated to be 4.46 and 7.75 from the titration curve (Figure
2d), corresponding to the proton dissociation of carboxyl group and
hydroxyl group on the backbone, respectively. The structural
difference at different pH values was also determined via UV−vis
absorption spectroscopy (Figure 2e). It shows that the absorption
peaks of 2,3-HCNQ at pH 7 and pH 14 in ultraviolet band are
similar, ascribed to the π−π* transition of the benzene ring.
However, the absorption peaks in the visible band appear at 409 and
472 nm, respectively, owing to the presence of different ionization
forms in neutral and alkaline solutions. Considering the adjacent
effect, we propose that an intramolecular hydrogen bond is formed
between the carboxylate anion and hydroxyl group at pH 7, while the
negatively charged carboxylate anion and phenol anion repelled each
other at pH 14 (Scheme S1a,b). To investigate the variation of
deprotonation degree of 2,3-HCNQ at different oxidation states, the
CV curves and the Pourbaix diagram at different pH values are
measured (Figure S5). The CV curves show that 2,3-HCNQ undergoes a
2H+− 2e− process over the whole pH range between 1.39−14. The
Pourbaix diagram is almost a straight line when the pH is
approaching to 14. This is different from previous reports of other
quinones,14,18 of which both oxidized and reduced forms are fully
deprotonated. These results indicate that the reduced form of
2,3-HCNQ may not be fully deprotonated in the alkaline solution,
probably owing to the formation of intramolecular hydrogen bonds
and the restraint of already- dissociated dianion structure (Scheme
S1c). With the introduction of the carboxyl group, the formal
potential of 2,3-HCNQ is negatively shifted by ∼30 mV compared to
2-HNQ (Figure S6a), which is abnormal for electron-withdrawing
groups.19 To understand this phenom- enon, we also measured the
UV−vis absorption spectrum of 2- HNQ in alkaline solution. Compared
to 2-HNQ, the absorption peak of 2,3-HCNQ in visible band is
positively shifted by ∼19 nm (Figure S6b). It indicates the
conjugated effect between the carboxylate group and the backbone of
naphthoquinone can effectively decrease the total molecule energy.
Meanwhile, the carboxyl group tends to lose the proton in alkaline
solution, thus contributing to the greatly improved solubility.
According to the quantitative analysis of UV−vis absorption spectra
(Figure S7), the solubility of 2,3-HCNQ in 1.0 M KOH is as high as
1.2 M at 20 °C, while the solubility of 2-HNQ in 1.0 M KOH is only
∼0.48 M, revealing the introduction of the carboxyl group can
significantly enhance the solubility in alkaline solution, which
confirms the importance of molecular structure design for ARFBs.
Notably, the stability of organic molecules has a great
effect
on the battery performance.13,18 First, the electrochemical
stability of 2,3-HCNQ was tested, as shown in Figure S8. The CV
curve of 2,3-HCNQ solution at the 50th cycle is nearly overlapped
with that at the first cycle. Moreover, the CV curve within a wide
potential range from 0.2 to −1.4 V vs Ag/AgCl shows no additional
peaks (Figure S8b), indicaing the absence of a side reaction and
the good electrochemical stability.
Moreover, the thermal stability of 2,3-HCNQ is also investigated by
thermogravimetric analysis (TGA), showing that 2,3-HCNQ can
withstand elevated temperature up to 160 °C with no obvious mass
loss (Figure S9). Subsequently, as the temperature progressively
increased to 250 °C, the carboxyl group is removed and converted to
CO2, which accounts for ∼20% weight of 2,3-HCNQ (Mr: 217.95 g
mol−1, see Figure S4). When the temperature is higher than 250 °C,
the naphthoquinone backbone will be gradually decomposed and
carbonized. Thus, it is expected that 2,3-HCNQ can easily endure
harsh temperatures. Considering the preparation conditions of ester
hydrolysis step (heated in 10% KOH solution at 60 °C overnight),
2,3-HCNQ is proved to be chemically stable in alkaline solution,
which is also conducive to the operation of ARFBs. The diffusion
coefficient (D) and rate constant (k0) of 2,3-
HCNQ were investigated by linear sweep voltammetry (LSV) method.
Figure 3a shows the RDE curves at various rotation
rates between 300 and 3000 rpm, and the well-defined plateau shapes
indicate the mass-transport controlled limiting currents. Figure 3b
illustrates the linear fitting of limiting currents against the
square roots of rotation rates measured at −1.3 V vs Ag/AgCl.
According to the Levich equation,17 the diffusion coefficient of
2,3-HCNQ is calculated to be 3.44 × 10−6 cm2
s−1 after background current calibration. Subsequently, a series of
Koutecky−Levich plots obtained at different overpotentials between
5 and 300 mV are depicted (Figure 3c). The intercepts to the y axis
represent the exchange currents (ik) at specific overpotentials.
The values of ik were transformed to logarithm form to illustrate
the Tafel plot (Figure 3d). Using the Butler−Volmer equation, the
k0 is calculated to be 2.07 × 10−3 cm s−1, which is higher than
that of common inorganic redox-active cations, such as VO2+/VO+,
V3+/V2+, and Fe3+/ Fe2+, implying that 2,3-HCNQ is a good candidate
for anolyte material for alkaline ARFBs.37
Figure 3. (a) RDE tests of 2 mM 2,3-HCNQ in 1.0 M KOH solution with
a scan rate of 25 mV s−1 at different rotation rates between 300
and 3000 rpm. (b) Levich plot of the limiting currents vs the
square roots of rotation rates. (c) Koutecky−Levich plots (the
reciprocals of currents vs the square roots of rotation rates)
derived from panel a at different overpotentials. (d) Fitting plot
of Butler−Volmer equation derived from panel c at different
overpotentials.
ACS Energy Letters Letter
The electrochemical performances of 2,3-HCNQ/K4Fe- (CN)6 ARFB were
systematically tested. The anolyte is 5.0 mL of 0.5 M 2,3-HCNQ in
2.0 M KOH solution, and the catholyte is 15.0 mL of 0.4 M K4Fe(CN)6
in 1.0 M KOH solution. Nafion 212 membrane was used as the
separator, and KOH-activated carbon paper with large active surface
area was used as the electrode (Figure S10). The CV curves of 2 mM
K4Fe(CN)6 and 2 mM 2,3-HCNQ solutions in 1.0 M KOH were measured
separately, predicting a cell potential of 1.02 V (Figure 4a). To
make sure the cell potential does not deviate
from the charge−discharge plateaus owing to the possible
rearrangement of organic redox-active species,11,20,28 we used the
galvanostatic intermittent titration technique (GITT) to more
accurately evaluate the cell potential. Figure 4b shows the
corresponding open-circuit voltages obtained at different states of
charge (SOC). The open-circuit voltage at 10% SOC is 0.957 V. When
the SOC is 50%, the open-circuit voltage is recorded to be 1.022 V,
which is very close to the CV result. The voltage versus time
curves of the initial 10 cycles at a
current density of 100 mA cm−2 are shown in Figure 4c, indicating
good cycling stability and reproducibility. The initial cycle
time-anolyte volume ratio is calculated to be 272 s mL−1
cycle−1, which is almost the same as that of 2,6-DHAQ/ K4Fe(CN)6
ARFB,14 indicating a similar anolyte utilization ratio. Concretely,
the galvanostatic charge−discharge profiles
(Figure 4d) show that the average charge and discharge voltages of
the first cycle at 100 mA cm−2 are 1.19 and 0.84 V, respectively.
The charge plateau is well-maintained for 100 cycles, while the
discharge plateau is slightly decreased, likely because of the
activity decay of carbon paper electrodes, not because of the
increase of membrane impedance. The initial discharge capacity is
18.8 Ah L−1, which is 70% of the theoretical capacity (26.8 Ah
L−1), and shows only a small attenuation after 100 cycles. The
cycling performance of 2,3-HCNQ/K4Fe(CN)6 ARFB
is shown in Figure 4e. During the whole 100 cycles, the Coulombic
efficiency is well-maintained at nearly 100%, and the energy
efficiency is kept around 68.8%. The discharge capacity remains at
17.8 Ah L−1 after 100 cycles, corresponding to a retention ratio of
94.7%. The total charge−discharge time is ∼1.5 days, delivering a
temporal capacity fade rate of 3.4% day−1 and an average Coulombic
efficiency close to 100%. When the anolyte concentration was
increased to 1.0 M (corresponding to 2.0 M electron concentration),
the 2,3- HCNQ/K4Fe(CN)6 ARFB exhibited an initial discharge
capacity of 41.0 Ah L−1 (Figure S11), which is ∼76.5% of the
theoretical capacity limit at this concentration (53.6 Ah L−1). The
capacity of this cell decayed to 39.2 Ah L−1 after 20 cycles,
showing a temporal capacity fade rate of 6.4% day−1, possibly
because of the anolyte decomposition and the small leakage of the
cell. As mentioned above, 2-HNQ suffers from severe capacity
loss, possibly because of the interference of oxygen. For
comparison, we measured the performance of 2-HNQ/ K4Fe(CN)6 ARFB
with 0.25 M concentration of 2-HNQ (Figure S12). Because of the
lower solubility of 2-HNQ, higher concentration may result in the
blockage of flow channels during the cycling processes. The
2-HNQ/K4Fe(CN)6 battery operated at 100 mA cm−2 shows a temporal
fade rate of 4.1% day−1, which is slightly higher than that of
2,3-HCNQ/ K4Fe(CN)6 ARFB. These results indicate that the stability
of these two naphthoquinone derivatives is comparable with that of
2,6-DHAQ but higher than that of 2,5-dihydroxy-1,4- benzoquinone in
alkaline solution.14,18
Moreover, the symmetric flow cell based on 0.1 M 2,3- HCNQ solution
was also tested by galvanostatic mode (Figure S13), showing a
capacity retention of 94.1% after 50 cycles and a temporal capacity
fade rate of 7.3% day−1, comparable to the result of 2,6-DHAQ
tested by potentiostatic mode.38 The reasons for the different
temporal capacity fade rates of full cell and symmetric cell might
be due to the variations of decomposition rate, leakage rate,
asymmetric polarized resistance, etc. Besides, we have performed a
cycling pause at 0% SOC and 100% SOC of another symmetric cell
based on 0.1 M 2,3-HCNQ solution with a resting time of 24 h to
isolate the chemical stability of reduced and oxidized form of 2,3-
HCNQ (Figure S14). The symmetric cell exhibits a fade rate of 0.6%
day−1 and 13.4% day−1 at 0% SOC and 100% SOC at initial stage,
respectively, indicating that the reduced form of 2,3-HCNQ is not
very chemically stable, just like the other redox species.38
Certainly, it should be admitted that the structure of
naphthoquinones needs to be further optimized, similar to
hydroxyanthraquinones.39
To evaluate the power output performance, the polarization curve of
2,3-HCNQ/K4Fe(CN)6 ARFB was recorded at different SOCs by LSV
method (Figure 4f), because this method imposed minimal
perturbation to the charge state compared to point-by-point
galvanostatic holds.14 The peak
Figure 4. (a) CV curves of 2 mM 2,3-HCNQ (red trace) and 2 mM
K4Fe(CN)6 (black trace) at 100 mV s−1 in 1.0 M KOH. (b) Open-
circuit voltages of 2,3-HCNQ/K4Fe(CN)6 ARFB obtained at different
states of charge by GITT method. (c) Voltage vs time curves of
2,3-HCNQ/K4Fe(CN)6 ARFB at 100 mA cm−2 recorded between the 1st and
10th cycles. (d) Galvanostatic charge− discharge curves of
2,3-HCNQ/K4Fe(CN)6 ARFB during the 1st, 50th, and 100th cycles at
100 mA cm−2. (e) Cycling performance, Coulombic efficiencies, and
energy efficiencies of 2,3-HCNQ/ K4Fe(CN)6 ARFB at 100 mA cm−2. (f)
Polarization curves and areal power densities of the
2,3-HCNQ/K4Fe(CN)6 ARFB at 10%, 50%, and ∼100% SOC at 25 °C.
ACS Energy Letters Letter
power densities at 10%, 50%, and ∼100% SOCs are calculated to be
0.162, 0.211, and 0.255 W cm−2, respectively. The peak power
denisties measured here are higher than FMN/ K4Fe(CN)6 and
1,8-DHAQ/K4Fe(CN)6 ARFBs at room temperature,17,27 which is
ascribed to higher concentration of 2,3-HCNQ anolyte. In summary,
we propose that the rationally designed 2,3-
HCNQ molecule with good electrochemical properties and high
solubility is a promising candidate for anolyte material for
alkaline ARFBs. Benefitting from the rapid kinetic rates of both
catholyte and anolyte, the 2,3-HCNQ/K4Fe(CN)6 ARFB can be stably
operated at a current density of 100 mA cm−2 for more than 100
cycles without any noble metal electrocatalyst. Moreover, 2,3-HCNQ
is composed of earth-abundant elements and can be synthesized at
very low cost, which is beneficial for the sustainable use of the
society. It is also promising to combine the 2,3-HCNQ/K4Fe(CN)6
ARFB with other inexpensive, stable, and highly ion-selective mem-
branes39,40 to further lower its capital cost for large-scale
energy storage applications.
ASSOCIATED CONTENT *S Supporting Information The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acsenergy- lett.8b01296.
Experimental section and additional figures and tables (PDF)
AUTHOR INFORMATION Corresponding Authors *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID Jie Liu:
0000-0003-0451-6111 Zhong Jin: 0000-0001-8860-8579 Author
Contributions ⊥C.W. and Z.Y. contributed equally. Notes The authors
declare no competing financial interest.
ACKNOWLEDGMENTS This work is supported by National Key R&D
Program of China (2017YFA0208200, 2016YFB0700600, and 2015CB659300)
, Pro jects o f NSFC (21872069 , 51761135104, 21573108, and
21332005), High-Level En- trepreneurial and Innovative Talents
Program of Jiangsu Province, and the Fundamental Research Funds for
the Central Universities (020514380146).
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ACS Energy Letters Letter
ACS Energy Letters Letter